In one of the world's most dangerous countries Diego Murillo may be one of its most feared men.

The Colombian warlord, nicknamed the Terminator, is accused of being a major drug smuggler and of murdering a provincial politician, his sister and their driver. He is also wanted in the US on charges of cocaine trafficking.

But Mr Murillo, 44, is the linchpin in talks between the 15,000 rightwing paramilitaries of the United Self-Defence Forces (AUC), which he leads, and the goverment which wants them to disarm.

Last week Mr Murillo, wearing a poncho with the Colombian flag, was flown by helicopter to a field near the town of Valencia in the northern province of Cordoba to watch as 450 of his fighters handed in their weapons.

It was a symbolic move and may lead to lighter sentences for him over the killings of which he is accused.

But that is a prospect which has angered human rights groups, who yesterday criticised legislation passed on Wednesday which could lead to generous concessions to paramilitary leaders.

With Mr Murillo apparently calling the shots from the house where he is being held by police, pressure has been mounting on Álvaro Uribe, the president, to take a tougher stance.

To human rights groups, Mr Murillo's case typifies the way paramilitary leaders became involved in drug trafficking and are trying to escape justice.

He began his criminal career washing cars for gangsters, rising to become a bodyguard for a member of the Medellin cartel then run by Pablo Escobar.

With his trademark brutality, he rose through the ranks of the cartel but Mr Murillo, then known as Don Berna, eventually turned on Escobar, sparking a feud during which hundreds of people died.

After Escobar's death, Mr Murillo was thought be have masterminded a number of high-profile killings and with hundreds of millions of drug dollars at his disposal, he crushed most of his rivals to become one of the most important paramilitary figures.

Although he has tried to clean up his image, writing articles with titles such as The Children: Our Constant Preocupation, he is still wanted by international anti-narcotics agencies.

Last year, the US included him in its list of major drug traffickers and said it wanted him extradited to stand trial for importing thousands of kilos of cocaine to the US. He denies the charges.

Distaste for the way in which the government is dealing with Mr Murillo has undermined confidence in the peace process and public support is being further diminished by reports of hundreds of infractions of the ceasefire which ended four decades of civil war.

The government concedes that paramilitaries continue to kill opponents and against this backdrop the congress has been debating how to deal with gunmen who disarm. If the punishments are too severe, the paramilitaries have said there will be no incentive to surrender and they will withdraw from talks.

On Wednesday, congress approved "The Justice and Peace Law", that could lead to thousands of fighters being disarmed. But their paramilitary commanders will not face serious punishment, or extradition to the US.

It is expected to be signed into law by President Uribe within days.

"This is a law that brings no justice, no peace," Senator Jimmy Chamorro told the New York Times. "It should be called what it really is, a law of impunity and immunity."

"This gives benefits to people who have committed the worst crimes, and we get nothing in return," said Gina Parody, a congresswoman and ally of President Uribe who nevertheless proposed much tougher legislation.

Human rights groups have long feared the government will strike a deal which will allow the druglords to remain free and rich.

José Vivanco, director of Human Rights Watch Americas, said the deal will "launder the criminal records of the top paramilitary commanders - including some of the most powerful drug lords - while allowing them to keep their wealth and maintain control over much of the country."

Meanwhile, the paramilitaries and their supporters in congress have condemned Mr Murillo's detention, but have pledged to continue talks with the government.